Thermally and electrically conductive structure, method of applying a carbon coating to same, and method of reducing a contact resistance of same

ABSTRACT

A thermally and electrically conductive structure comprises a carbon nanotube ( 110 ) having an outer surface ( 111 ) and a carbon coating ( 120 ) covering at least a portion of the outer surface of the carbon nanotube. The carbon coating may be applied to the carbon nanotube by providing a nitrile-containing polymer, coating the carbon nanotube with the nitrile-containing polymer, and pyrolyzing the nitrile-containing polymer in order to form the carbon coating on the carbon nanotube. The carbon nanotube may further be coated with a low contact resistance layer ( 130 ) exterior to the carbon coating and a metal layer ( 140 ) exterior to the low contact resistance layer.

CLAIM OF PRIORITY

This application is a continuation of U.S. patent application Ser. No.13/852,343, filed on Mar. 28, 2013, which was a continuation of U.S.patent application Ser. No. 13/371,575, now U.S. Pat. No. 8,409,665,filed on Feb. 13, 2012, which was a divisional of U.S. patentapplication Ser. No. 12/587,371, now U.S. Pat. No. 8,133,585, filed onOct. 5, 2009, which was a divisional of U.S. patent application Ser. No.11/845,145 now U.S. Pat. No. 7,618,679, filed on Aug. 27, 2007.

FIELD OF THE INVENTION

The disclosed embodiments of the invention relate generally tomicroelectronic devices and packages, and relate more particularly tocarbon nanotubes used in such microelectronic devices and packages.

BACKGROUND OF THE INVENTION

Carbon nanotubes have high current carrying capacity as well as highthermal conductivity. However, their use in semiconductor packages islimited due to electron and phonon scattering occurring at theirinterfaces, leading to high electrical and thermal resistances. A majorchallenge in overcoming such limitations lies in tailoring the interfaceto control interface resistance. Typically it is proposed that thenanotube surface should be functionalized with low interface resistancematerials, or that a low contact resistance material such as titanium ornickel should be sputtered onto the ends of a carbon nanotube.Functionalization, however, introduces defects in the nanotube structureand may deteriorate its intrinsic thermal and electrical properties,while sputtering with low contact resistance materials may not alwaysachieve good bonding with nanotubes, especially if the sputtered metalis not a carbide former. Alternate methods of tailoring the nanotubeinterface are therefore necessary, methods that form good interfacialadhesion and lower contact resistance at nanotube surfaces withoutcompromising the nanotubes' desirable intrinsic properties.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed embodiments will be better understood from a reading ofthe following detailed description, taken in conjunction with theaccompanying figures in the drawings in which:

FIG. 1 is a cross-sectional view of a thermally and electricallyconductive structure according to an embodiment of the invention;

FIG. 2 is a flowchart illustrating a method of applying a carbon coatingto a carbon nanotube according to an embodiment of the invention;

FIG. 3 is a representation of the pyrolysis of poly(acrylonitrile) intoa graphitic form suitable for use as a coating on a carbon nanotubeaccording to an embodiment of the invention;

FIG. 4 is a flowchart illustrating a method of reducing a contactresistance of a carbon nanotube according to an embodiment of theinvention;

FIG. 5 is a representation of a polymer that may be used in theformation of a carbon coating according to an embodiment of theinvention; and

FIG. 6 is a schematic representation according to an embodiment of theinvention showing polar nitrile segments of the polymer of FIG. 5interacting with acidified carbon nanotubes.

For simplicity and clarity of illustration, the drawing figuresillustrate the general manner of construction, and descriptions anddetails of well-known features and techniques may be omitted to avoidunnecessarily obscuring the discussion of the described embodiments ofthe invention. Additionally, elements in the drawing figures are notnecessarily drawn to scale. For example, the dimensions of some of theelements in the figures may be exaggerated relative to other elements tohelp improve understanding of embodiments of the present invention. Thesame reference numerals in different figures denote the same elements.

The terms “first,” “second,” “third,” “fourth,” and the like in thedescription and in the claims, if any, are used for distinguishingbetween similar elements and not necessarily for describing a particularsequential or chronological order. It is to be understood that the termsso used are interchangeable under appropriate circumstances such thatthe embodiments of the invention described herein are, for example,capable of operation in sequences other than those illustrated orotherwise described herein. Similarly, if a method is described hereinas comprising a series of steps, the order of such steps as presentedherein is not necessarily the only order in which such steps may beperformed, and certain of the stated steps may possibly be omittedand/or certain other steps not described herein may possibly be added tothe method. Furthermore, the terms “comprise,” “include,” “have,” andany variations thereof, are intended to cover a non-exclusive inclusion,such that a process, method, article, or apparatus that comprises a listof elements is not necessarily limited to those elements, but mayinclude other elements not expressly listed or inherent to such process,method, article, or apparatus.

The terms “left,” “right,” “front,” “back,” “top,” “bottom,” “over,”“under,” and the like in the description and in the claims, if any, areused for descriptive purposes and not necessarily for describingpermanent relative positions. It is to be understood that the terms soused are interchangeable under appropriate circumstances such that theembodiments of the invention described herein are, for example, capableof operation in other orientations than those illustrated or otherwisedescribed herein. The term “coupled,” as used herein, is defined asdirectly or indirectly connected in an electrical or non-electricalmanner. Objects described herein as being “adjacent to” each other maybe in physical contact with each other, in close proximity to eachother, or in the same general region or area as each other, asappropriate for the context in which the phrase is used. Occurrences ofthe phrase “in one embodiment” herein do not necessarily all refer tothe same embodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

In one embodiment of the invention, a thermally and electricallyconductive structure comprises a carbon nanotube having an outer surfaceand a carbon coating covering at least a portion of the outer surface ofthe carbon nanotube. The carbon coating may be applied to the carbonnanotube, in one embodiment, by providing a nitrile-containing polymer,coating the carbon nanotube with the nitrile-containing polymer, andpyrolyzing the nitrile-containing polymer in order to form the carboncoating on the carbon nanotube.

The application of the carbon coating may assist in the reduction ofcontact resistance for the carbon nanotube and may further provide areactive surface that can be utilized to further extend the carbonnanotube to applications in either hydrophobic or hydrophilicenvironments. As an example, the carbon coating may act as a site forthe attachment or coating of materials that reduce thermal or electricalcontact resistance between the nanotube and other materials. To thatend, a low contact resistance layer may be formed exterior to the carboncoating and a metal layer may be formed exterior to the low contactresistance layer. These processes do not alter the nanotube's structureand therefore do not disturb the intrinsic electrical and thermalproperties of the nanotube.

Referring now to the drawings, FIG. 1 is a cross-sectional view of athermally and electrically conductive structure 100 according to anembodiment of the invention. As illustrated in FIG. 1, thermally andelectrically conductive structure 100 comprises a carbon nanotube 110having an outer surface 111 and further comprises a carbon coating 120covering at least a portion of outer surface 111 of carbon nanotube 110.Carbon nanotube 110 can be a single walled carbon nanotube (SWCNT), adouble walled carbon nanotube (DWCNT), or another multi-walled carbonnanotube (MWCNT).

In one embodiment carbon coating 120 surrounds substantially all ofouter surface 111 of carbon nanotube 110. In the same or anotherembodiment, a structure of carbon coating 120 is that of a graphitesheet. In a manner that will be further described below, carbon coating120 may be derived from a nitrile-containing polymer.

Carbon coating 120 comprises a reactive surface 121 having danglingcarbon bonds to which may be attached materials that reduce thermaland/or electrical contact resistance between carbon nanotube 110 andstill other materials that may be desirable in a microelectronicsenvironment. In FIG. 1, a low contact resistance layer 130 is locatedexterior to carbon coating 120 and interior to a metal layer 140. Lowcontact resistance layer 130 is an example of a material that reducesthermal and/or electrical contact resistance between carbon nanotube 110and metal layer 140. In one embodiment low contact resistance layer 130comprises a carbide-forming metal (which has a high affinity to carbon).In another embodiment low contact resistance layer 130 comprises amatrix-compatible polymer, where the matrix could include metal layer140 or a polymer or the like.

The electrical contact resistance of carbon nanotubes may be lowered byapplying a coating of carbon paste or carbon black around the carbonnanotubes because carbon paste is composed of small graphite sheetswhich would stack onto the carbon nanotubes. This carbon paste or carbonblack may form carbon coating 120. Due to the similarity in electronicstructure between carbon nanotubes and these carbon coatings one wouldexpect low electrical contact resistance between the two materials.

To a first order, thermal contact resistance is a function of latticemismatch at the interface; the greater the mismatch the worse is thethermal resistance. Carbon coating 120 may have lattice parameterssimilar to those of carbon nanotubes 110, in that both contain thestructure of graphite sheets. Thus, one would expect the carbon coatingto reduce the thermal interface resistance across the nanotube surface.

In a non-illustrated embodiment of the invention low contact resistancelayer 130 may be omitted and carbon coating 120 may be immediatelyadjacent to metal layer 140. However, the presence of low contactresistance layer 130 may create a smooth gradient of properties betweencarbon nanotube 110 and metal layer 140 without disturbing the intrinsicstructure and properties of carbon nanotube 110. Because of its role insmoothly bridging a gap between carbon nanotube properties and matrixmetal (or polymer) properties, low contact resistance layer 130 may alsobe referred to at times herein as a transition layer. Alternatively,another metal layer or polymer layer or the like having properties thatlie between those of carbon nanotubes and matrix materials can act assuch a transition layer. As an example, the transition layer cancomprise copper, nickel, ruthenium, platinum, or the like. It should benoted that carbon coating 120 also acts as a transition layer in that ittoo has properties that bridge a gap between carbon nanotubes and matrixmaterials, but carbon coating 120 is referred to herein with its ownname and reference numeral in order to make it clear that it is adistinct layer separate from low contact resistance layer 130 (oranother transition layer).

A wide variety of nitrile-containing polymers can be utilized to obtaincarbon coating 120, including polyacrylonitrile, poly(phthalonitrile),copolymers such as poly(styrene)-b-poly(phthalonitrile) and the like. Asfurther discussed below, the initial polymer stabilization can beachieved by bulk dispersion of the polymer in solvent via wet chemistrytechniques or by controlled stabilization methods utilizing colloidchemistry techniques where the nanotubes must first be acidified andthen stabilized.

FIG. 2 is a flowchart illustrating a method 200 of applying a carboncoating to a carbon nanotube according to an embodiment of theinvention. A step 210 of method 200 is to provide a nitrile-containingpolymer. As an example, the nitrile-containing polymer can includepoly(acrylonitrile), poly(phthalonitrile), copolymers such aspoly(styrene)-b-poly(phthalonitrile), block copolymer polymerstabilizers such as poly(styrene-b-4-vinylphenoxyphthalonitrile), andthe like. More generally, any polymer containing a nitrile group may beused.

A step 220 of method 200 is to perform an acid treatment on the carbonnanotube in order to enhance the adsorption of the nitrile-containingpolymer to the carbon nanotube. Step 220 is an optional step that may beomitted in certain embodiments of method 200 and for certain polymers.Step 220 or another step may further comprise centrifuging and washingthe carbon nanotube and then drying it in order to remove anynon-adsorbed polymer.

A step 230 of method 200 is to coat the carbon nanotube with thenitrile-containing polymer. In one embodiment, step 230 comprisesdispersing the nitrile-containing polymer in a solvent such that thenitrile-containing polymer is adsorbed to the carbon nanotube. In thisembodiment polymers such as homopolymers and random copolymerscontaining acrylonitrile/phthalonitrile may be used to generategraphitic coating on carbon nanotubes by bulk dispersion of polymer insolvent. The adsorption of the polymer to the carbon nanotube can beenhanced by acid treatment as in step 220.

In another embodiment, step 230 comprises electrospinning thenitrile-containing polymer and the carbon nanotube in order to create ananofiber composite. In an embodiment where the nitrile-containingpolymer comprises a block copolymer, step 230 can comprise acidifyingthe carbon nanotube and attaching the block copolymer to the carbonnanotube using a colloidal chemistry method. As an example, the polymerscan be block copolymers such as poly(styrene)-b-poly(phthalonitrile).The carbon nanotubes may be acidified as in step 220 and then coatedwith the block copolymers by colloidal chemistry methods such as micelleformation, where the polar nitrile components complex the COOH (or otheracid) groups on the carbon nanotube surface. When nonpolar solvent isadded to the mixture the polystyrene pultrudes out into the nonpolarsolvent.

A step 240 of method 200 is to pyrolyze the nitrile-containing polymerin order to form the carbon coating on the carbon nanotube. Thepyrolysis of step 240 converts the polymer coating to a carbon/graphiticcoating. In one embodiment, step 240 comprises pyrolyzing thenitrile-containing polymer at a temperature greater than 500 degreesCelsius. In a particular embodiment, step 240 comprises pyrolyzing thenitrile-containing polymer in a tube furnace or the like atapproximately 700 degrees Celsius for approximately six hours in argon.

FIG. 3 is a representation of the pyrolysis of poly(acrylonitrile) intoa graphitic form suitable for use as a coating on a carbon nanotubeaccording to an embodiment of the invention. Pyrolysis of other polymersproceeds in a similar manner. It should be noted that the graphiticsheets that result from the illustrated pyrolysis procedure have thesame structure that would be obtained by slicing a carbon nanotube openlengthwise and unrolling it. As discussed above, this structuralsimilarity between the carbon nanotubes and the graphitic sheets of thecarbon coating may produce low electrical contact resistance at thecarbon nanotube/carbon coating interface.

FIG. 4 is a flowchart illustrating a method 400 of reducing a contactresistance of a carbon nanotube according to an embodiment of theinvention. A step 410 of method 400 is to provide a nitrile-containingpolymer. As an example, and as stated above, the nitrile-containingpolymer can include poly(acrylonitrile), poly(phthalonitrile),copolymers such as poly(styrene)-b-poly(phthalonitrile), block copolymerpolymer stabilizers such aspoly(styrene-b-4-vinylphenoxyphthalonitrile), and the like. Moregenerally, any polymer containing a nitrile group may be used.

A step 420 of method 400 is to perform an acid treatment on the carbonnanotube in order to enhance the adsorption of the nitrile-containingpolymer to the carbon nanotube. Step 420 is an optional step that may beomitted in certain embodiments of method 400 and for certain polymers.Step 420 or another step may further comprise centrifuging and washingthe carbon nanotube and then drying it in order to remove anynon-adsorbed polymer.

A step 430 of method 400 is to coat the carbon nanotube with thenitrile-containing polymer. As an example, step 430 can proceedaccording to any of the embodiments described above in connection withstep 230 of method 200, or similar embodiments.

A step 440 of method 400 is to pyrolyze the nitrile-containing polymerin order to form a carbon coating on the carbon nanotube. The pyrolysisof step 440 converts the polymer coating to a carbon/graphitic coating.In one embodiment, step 440 comprises pyrolyzing the nitrile-containingpolymer at a temperature greater than 500 degrees Celsius. In aparticular embodiment, step 440 comprises pyrolyzing thenitrile-containing polymer in a tube furnace or the like at atemperature of not more than approximately 700 degrees Celsius forapproximately six hours in argon.

A step 450 of method 400 is to form a low contact resistance layerexterior to the carbon coating. As an example, the low contactresistance layer can be similar to low contact resistance layer 130 thatis shown in FIG. 1.

A step 460 of method 400 is to form a metal layer exterior to the lowcontact resistance layer. As an example, the metal layer can be similarto metal layer 140 that is shown in FIG. 1.

FIG. 5 is a schematic representation of a polymer 500 that may be usedin the formation of carbon coating 120 according to an embodiment of theinvention. Polymer 500 is a block copolymer polymer stabilizerpoly(styrene-b-4-vinylphenoxyphthalonitrile). A first (unbracketed)block of polymer 500 is a nonpolar polystyrene segment. A second blockof polymer 500 (indicated by brackets 510) is a polar block segmentcontaining nitrile groups, as indicated. Polymer 500 is shown insimplified form on the right-hand side of FIG. 5, where two major blocksof polymer 500 are depicted. A block 501 corresponds to the first(unbracketed) block, while a block 502 corresponds to the second block(with brackets 510) containing the nitrile group. (The different linewidths for blocks 501 and 502 are merely an artifice introduced in orderto help distinguish the blocks from each other.)

FIG. 6 is a schematic representation according to an embodiment of theinvention showing polar nitrile segments (blocks 502) of polymer 500interacting with an acidified carbon nanotube. Following pyrolysis, thecarbon-coated carbon nanotubes may then be further coated with a lowcontact resistance/transition layer and/or a metal/polymer matrix layer,as explained above.

Although the invention has been described with reference to specificembodiments, it will be understood by those skilled in the art thatvarious changes may be made without departing from the spirit or scopeof the invention. Accordingly, the disclosure of embodiments of theinvention is intended to be illustrative of the scope of the inventionand is not intended to be limiting. It is intended that the scope of theinvention shall be limited only to the extent required by the appendedclaims. For example, to one of ordinary skill in the art, it will bereadily apparent that the thermally and electrically conductivestructure, the methods of applying a carbon coating to a carbonnanotube, and the methods of reducing a contact resistance of a carbonnanotube discussed herein may be implemented in a variety ofembodiments, and that the foregoing discussion of certain of theseembodiments does not necessarily represent a complete description of allpossible embodiments.

Additionally, benefits, other advantages, and solutions to problems havebeen described with regard to specific embodiments. The benefits,advantages, solutions to problems, and any element or elements that maycause any benefit, advantage, or solution to occur or become morepronounced, however, are not to be construed as critical, required, oressential features or elements of any or all of the claims.

Moreover, embodiments and limitations disclosed herein are not dedicatedto the public under the doctrine of dedication if the embodiments and/orlimitations: (1) are not expressly claimed in the claims; and (2) are orare potentially equivalents of express elements and/or limitations inthe claims under the doctrine of equivalents.

What is claimed is:
 1. A method of reducing a contact resistance of acarbon nanotube, the method comprising: coating the carbon nanotube withthe nitrile-containing polymer; pyrolyzing the nitrile-containingpolymer in order to form a carbon coating on the carbon nanotube;forming a carbide-forming metal layer exterior to the carbon coating;and forming a metal layer exterior to the carbide-forming metal layer.2. The method of claim 1, wherein pyrolyzing the nitrile-containingpolymer comprises pyrolyzing the nitrile-containing polymer at atemperature greater than 500 degrees Celsius.
 3. The method of claim 2,wherein pyrolyzing the nitrile-containing polymer comprises pyrolyzingthe nitrile-containing polymer in a tube furnace at approximately 700degrees Celsius for approximately six hours in argon.
 4. The method ofclaim 1, wherein coating the carbon nanotube comprises dispersing thenitrile-containing polymer in a solvent such that the nitrile-containingpolymer is adsorbed to the carbon nanotube.
 5. The method of claim 1,further comprising performing an acid treatment on the carbon nanotube.6. The method of claim 1, wherein coating the carbon nanotube compriseselectrospinning the nitrile-containing polymer and the carbon nanotubein order to create a nanofiber composite.
 7. The method of claim 1,wherein coating the carbon nanotube with the nitrile-containing polymercomprises: acidifying the carbon nanotube; and attaching thenitrile-containing polymer, comprising a block copolymer, to the carbonnanotube using a colloidal chemistry method.
 8. A method of reducing acontact resistance of a carbon nanotube, the method comprising: coatingthe carbon nanotube with a nitrile-containing polymer; pyrolyzing thenitrile-containing polymer in order to form a carbon coating on thecarbon nanotube; forming a polymer layer exterior to the carbon coating;and forming a metal layer exterior to the polymer layer.
 9. The methodof claim 8, wherein pyrolyzing the nitrile-containing polymer comprisespyrolyzing the nitrile-containing polymer at a temperature greater than500 degrees Celsius.
 10. The method of claim 9, wherein pyrolyzing thenitrile-containing polymer comprises pyrolyzing the nitrile-containingpolymer in a tube furnace at approximately 700 degrees Celsius forapproximately six hours in argon.
 11. The method of claim 8, whereincoating the carbon nanotube comprises dispersing the nitrile-containingpolymer in a solvent such that the nitrile-containing polymer isadsorbed to the carbon nanotube.
 12. The method of claim 8, furthercomprising performing an acid treatment on the carbon nanotube.
 13. Themethod of claim 8, wherein coating the carbon nanotube compriseselectrospinning the nitrile-containing polymer and the carbon nanotubein order to create a nanofiber composite.
 14. The method of claim 8,wherein coating the carbon nanotube with the nitrile-containing polymercomprises: acidifying the carbon nanotube; and attaching thenitrile-containing polymer, comprising a block copolymer, to the carbonnanotube using a colloidal chemistry method.